1. Field of the Invention
The invention relates to a process and a device for sealing the joint plane between two joined refractory parts, for example a joint plane in a molten metal flow conduit.
2. Discussion of the Background
In order to cast steel from a ladle into an ingot mold, the steel is tapped at the bottom of the ladle through a flow-control slide valve, then flows into a distributor through a gate-protecting tube; the steel is then tapped at the bottom of the distributor through an internal nozzle, after which it flows through an external nozzle and into the ingot mold.
Along the steel casting line, there are then disposed two Joint planes of joined refractory parts: one joint plane between two refractory plates of the ladle-tapping slide valve and one joint plane between the two internal and external refractory nozzles.
In practice, a ladle-tapping flow-control slide valve is generally provided with two joined refractory plates sliding one over the other in a plane perpendicular to the direction of flow of the molten metal, each perforated by a hole; the flow of steel can be controlled by sliding the plates in order to adjust the covered areas of the two holes
Documents FR 2,560,085, FR 2,415,507 and FR 2,529,493, all incorporated herein by reference describe tapping flow-control slide valves:
in FR 2,560,085, (which corresponds to U.S. Pat. No. 4,721,236) there are formed, close to the joint plane of the refractory plates, cavities in which hydrocarbons (for example: solid pitch, grease, or methane gas) are disposed or injected: these hydrocarbons travel through the pores of the refractory, especially up to the joined surfaces of the plates, and "thus largely prevent the penetration of molten steel between the plates" (sealing effect) "while imparting a lubricating effect, which prevents deterioration due to friction" (page 2, lines 24 to 30). PA1 in FR 2,415,507, (which corresponds to U.S. Pat. No. 4,199,087) an inert gas (argon) is injected via an annular duct made in the joint plane of refractory parts forming a casting conduit in order to control the casting flow of metal (especially in order to keep the effect of excessive speed at the start of casting under control); the injected gas is located in the casting conduit. FIG. 3 of FR 2,415,507 shows an enlarged view of a tapping flow-control slide valve having a pouring tube and tube support plate which includes refractory parts 34 (tube support plate) and 25 (pouring tube) joined to form a portion of a conduit 54, an elongated passage 74 and groove 71 in the support plate acting as a fluid circulator for the gas, a gas injector 72, a metal shim 76 having an opening 78 and a set screw 70. PA1 in FR 2,529,493, it is recalled that the refractory plates of these slide valves are generally made of alumina, zirconia or magnesia, generally graphited, and are carefully impregnated with tar under vacuum; it is pointed out that the movement of metal in the casting tube causes air to be sucked between the plates and thus leads to substantial degradation of inclusion cleanliness of the cast metal; there is described a device provided with an annular duct encircling the casting tube at the position of the joint plane between the refractory parts ("zone to be lubricated and protected"); to improve sealing quality and to limit suction of air, "lubricating and protective material" (for example: pitch derived from coal or petroleum distillation) is injected into this annular duct; this material "expands in the space between the plates" (page 4, line 18). PA1 the cracking or decomposition temperature of the oil is lower than the temperature of the said joint plane at the position where the said inert gas is injected; consequently, the solid residues of cracking or decomposition plug up the said joint plane, thus improving the sealing thereof, PA1 the oil contains particles of carbon, especially of graphite and/or carbon black; these solid particles reinforce the effect of plugging of the joint plane. PA1 the oil is injected intermittently in "doses" of predetermined volume; the inert gas thrusts each dose of oil into the joint plane; the risks of obstruction of the gas-injection lines are reduced still further. PA1 if the gas is injected into an annular duct made around the conduit in the joint plane, the said volume of the oil dose is preferably larger than the volume of the annular duct. PA1 the injection or thrusting of oil is controlled as a function of the evaluation of the quality of sealing of the said joint plane; it is even possible to stop the injection of oil completely when it appears that a sufficient sealing quality has been achieved and to restart the injection of oil when the sealing quality deteriorates once again; this control makes it possible to maintain the consumption of inert gas at a minimum level and also makes it possible to limit the risks of obstruction. PA1 the injection of oil is controlled by modifying the frequency of injection of the oil doses. PA1 the said sealing quality is evaluated as a function of the injection pressure of the inert gas or as a function of the injection flowrate of the inert gas: at given flowrate, a low pressure is indicative of poor sealing; at given pressure, a high flowrate is indicative of poor sealing. PA1 at least two successive and joined refractory parts (34, 25) forming a portion of conduit (54) for the molten metal, PA1 a fluid circulator (74) made in at least one of the refractory parts (34) to extend as far as the joint plane (76, 78) of the said two refractory parts, PA1 an inert gas injector (72) adapted to inject inert gas into said circulator, PA1 the circulator and the gas injector being adapted to prevent introducing gas from the atmosphere into the said conduit at the position of the said joint plane, PA1 characterized in that it is provided with an oil injector or thrustor (73) for injecting or thrusting the oil into the said gas circulation means.
The joined surface of two internal and external refractory nozzles poses the same problems of sealing as the two joined refractory plates of a tapping slide valve; since the external nozzle, of refractory material, must be able to be changed in the course of casting (by means of a nozzle-changing device), this nozzle therefore shares a joint plane or joined surface with the internal nozzle.
If sealing between two joined refractory elements of the molten metal line is not sufficiently assured, the reduced pressure created in the casting conduit by the flow of molten steel causes suction of external gas at the position of the joint planes of the refractory parts of the line.
In order to prevent certain atmospheric gases such as oxygen and nitrogen from penetrating into the molten metal line at these points, an inert gas such as argon is generally injected at the position of the joined surface; the injected gas is therefore located in the molten metal line.
Thus at the position of the joint planes or joined surfaces of successive refractory parts of a molten metal casting line, there is generally provided an annular duct for diffusion of inert gas to encircle the molten metal conduit at that location.
This annular duct is connected to a supply conduit discharging toward the outside of the nozzles; it is connected in turn to means for inert gas injection.
During molten metal casting, a flow of inert gas intended to be sucked at the position of the joint planes of the refractory parts of the said line is therefore maintained in this annular duct, in such a manner that infiltrations of gas from the atmosphere into the molten metal line is prevented at this location.
The inert gas flowrate is even sufficiently high in general that a portion of the inert gas even escapes to the outside of the joint plane, or in other words toward the atmosphere.
The disadvantage of such a process is that it consumes large quantities of inert gas.
Another disadvantage is caused by the inert gas in the molten metal flow: this gas can have detrimental effects on solidification of the metal (especially in the ingot mold), which is harmful to the quality of the metal obtained.
In addition, to improve the sealing of the joint plane and to limit suction of air into the casting tube, a material such as described in FR 2,529,493, incorporated herein by reference, can also be injected into the joint plane.
To achieve such injection, there are then used circulation means made in one of the refractory elements in such a manner as to extend as far as the joint plane or interface between the refractory elements, as well as means for dispatching this material through these circulation means.
The disadvantage of the device described in FR 2,529,493 is that it rapidly becomes obstructed and that, in the case of obstruction of the means for circulating the injected material, the risk again develops that air will be sucked in large quantities into the casting tube.